(some possible mild spoilers)
Moments of it are good, but overall it was disappointing.
It is a pointless sequel/continuation (like so many movies/books, trying to keep the money train running).
Perhaps due to the script form, it is too cursory to really develop the characters and situations. Often I got lost trying to figure out why something was happening/done/said or even what was going on.
There is a glaring error in the fight with “what’s her name” (I forget and I’m too lazy to go look it up – the villainess). She disarms Harry and he’s hiding. Albus manages to pop up and distract her, and Harry attacks. But there is never any indication of how or even that he got his or any other wand back. Did he dive for it while she was distracted? Did Albus send one flying into his hand?
After the great build-up of the series of Harry Potter books, this story/conflict resolution was too easy and simplistic.
At times, Harry seemed totally out of character. Most of the others seemed reasonable, but particularly when he is giving McGonagall orders – not buying it. Granted this is an older, stressed Harry, possibly even a bit jaded, but that sort of thing was just never in his character. I didn’t believe it, even acting as a “concerned father”.
I didn’t hate it, but I didn’t love it. Just “meh”. Shrug it off as read, set it aside and likely never read it again. (Whereas I’ve reread the original HP books several times.)
That’s not to say that I’m totally down on all additional things Harry Potter. Fantastic Beasts looks like it is interesting, and could be very good. If that proves true, the fresh take on this world will prove preferable than trying to re-energize a finished story in the original books with additional “sequels”. The “snapshots” on Pottermore of various aspects of the wizarding world can be interesting and entertaining, but The Cursed Child just fell flat, in my opinion. Still, others love it, so I guess it depends on individual taste.